Actually you can use R-15 in your walls. Most builders use R-13 but the builder I work for ( I am a field super./const mgr ) we use R-15 in all our 2x4 walls. It really helps hold in the heat. You can use the foam on all your corners and around any door or window headers/jacks to do what is referred to as an air stop. Some call it chinking. They also make a caulking for that purpose.
If you have trusses where you can create a small attic space then a loose blown in insulation R-38 is prefect ( actually under the new IRC building codes all houses have gone to R-38 in attics or the type.)
I, like green man, building is my business. Any help you need just let me know.
The more you insulate and air stop the less heating you will have to worry about and will save you $$ in the long run.
Can't wait to see some pictures.